UK Phone Roam

With the recent news that EU wide phone roaming charges are to come down, I would like to ask why the major networks in the UK can't get their collective acts together for UK phone roaming. This would mean that in areas covered by one network and not another, my phone would switch networks to ensure I remained in coverage. The alternative is to muck about with multiple sim cards, which is hardly a solution.

Why is this apparantly so difficult?

Craig